When viewers are asked by newspapers or the BBC Trust to define what they would like to see more of on British television, the answer is invariably the same: more originality and less reliance on familiar formats. This instruction is then handed on to commissioners, who will soon be accused of having ignored it. But, if the schedules are accused of lacking this quality, it may be because the concept of originality in television is so complicated.

This weekend, the two main terrestrial networks launch their most expensive and publicised projects of the spring: The Voice (Saturday 7pm) on BBC1 and Titanic (Sunday 9pm) on ITV1. Yet, though brand new, both shows feel already curiously recognisable. The former is a singing contest which is clearly the corporation's latest attempt (after the calamitous Fame Academy) to match The X Factor. The latter not only revisits maritime history popularised in James Cameron's 1997 film but is also the same blend as Julian Fellowes's drama Downton Abbey only with added water: sirs and servants, scandalous daughters, English aristocrats and American plutocrats, upper deck and lower deck rather than upstairs and downstairs.

So, according to the prejudice shown in polls of viewers, these high-profile shows should be doomed by repeating a recipe. And yet history suggests that viewers' knowledge of what to expect will increase rather than reduce the programmes' chances of success.

Certainly, Downton Abbey, although now a vastly profitable franchise, initially suffered from criticisms almost identical to those being directed at Titanic. The country house drama, it was widely pointed out, had already been done on the big screen by Fellowes himself (Gosford Park) and on the small by Upstairs, Downstairs. Indeed, it was a popular critical trope to deride the new ITV nobs-and-slobs series as a tired remake of the old one. However, it proved to be such a success that it overshadowed the BBC's actual tired remake of Upstairs, Downstairs.

Downton joined schedules in which two of the biggest hits – Strictly Come Dancing and Britain's Got Talent – would be entirely familiar to a viewer who suddenly tuned in again after switching off their set in the 1970s: as variations on Come Dancing and Opportunity Knocks.

The reason for this may be that TV has reproduction deep in its DNA. For example, small-screen drama started by aping stage plays and then, in the 80s with the creation of Channel 4's film arm, moved towards pseudo-movies. Television's original variety and comedy shows so closely followed the model of music hall that it was standard for acts such as Morecambe & Wise and Tommy Cooper still to work in front of a curtain, although this had no technical purpose. Even the forms that the medium pioneered – current affairs discussions, sitcoms – have remained essentially unchanged over six decades or so. The ghosts of Richard Dimbleby and Lucille Ball would still recognise the genres they helped to pioneer.

What's depressing is how few television programmes make use of possibilities unique to this art-form. Big Brother was entirely shaped by and for the flow of 24-hour programming. John Hopkins's Talking to a Stranger (BBC, 1966) was the first of several dramas to exploit TV's potential for exploring the same events from different perspectives on different nights – the latest being next week's BBC drama One Night and, as it happens, Titanic.

Alan Bennett's one-person plays Talking Heads were technically revolutionary in combining the traditions of newscasters and theatrical monologuists and challenging the convention that filmed drama required a succession of cuts and characters. Generally, though, most innovations in TV are tonal rather than structural: Channel 4's Peep Show and BBC3's Being Human are fundamentally flat-share sitcoms but are innovative in their attitude to character, language and camera angle.

Having a job that often involves watching TV and theatre on the same day, I'm struck by the fact that, though rare in either medium, the shock of the absolutely new is found more often on the stage: watching Stephen Sondheim's Assassins or Pacific Overtures, Martin Crimp's Attempts on Her Life, Stewart Lee's and Richard Thomas's Jerry Springer: The Opera and Alecky Blythe's and Adam Cork's London Road, I felt in the presence of radical originality, whereas even the best TV can frequently seem, like poetry, to be a brilliant re-animation of existing forms.

This may be because theatre has a tradition of fringe and studio spaces, in which experimentation is encouraged and box office expectations lowered, which TV has only recently begun to provide – and, even then, accidentally – through digital expansion. A populist medium judged immediately on ratings, television requires too many shows to win a big audience quickly, resulting, as in mainstream Hollywood, in a reliance on remakes. Accordingly – as Titanic and The Voice again suggest – TV often favours deja vu.